<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 误区：通常认为元素的border，是由四个矩形边框拼接而成 */
        .d1{
            width: 100px;
            height: 100px;
            border: 5px solid red;
        }
        /* 实际上，元素的border，是由三角形拼接而已的 */
        .d2{
            /* width: 50px;
            height: 50px; */
            width: 0;
            height: 0;
            /* border-top: 10px solid red;
            border-right: 10px solid blue; 
            border-bottom: 10px solid green;
            border-left: 10px solid pink; */
            /* border-top: 100px solid red; */
            border-right: 100px solid transparent; 
            border-bottom: 100px solid green;
            border-left: 100px solid transparent;
        }
    </style>
</head>
<body>
    <div class="d1"></div>
    <hr>
    <div class="d2"></div>
</body>
</html>